Sauropterygians

Sauropterygia ("lizard flippers") is an extinct, diverse taxon of aquatic reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Mesozoic before they became extinct at the end of that era. At the beginning of the Cretaceous only a single lineage, the Xenopsaria, represented by the two groups Elasmosauridae and Polycotylidae, was still present. Sauropterygians are united by a radical adaptation of their pectoral girdle, designed to support powerful flipper strokes....